Other valence isomerizations

The special case of perfluorinated Dewar benzenes and heteroaromatic analogs is discussed in Section 5.3.4., together with other valence isomerizations of six-membered and larger rings. 

Despite the increasing information on the photochemistry of 2,4-dienones and other unsaturated ketones, as well as on the ring-chain valence isomerism of halogen-substituted pyran and dihydi opyran systems,the data are still very scarce. The intermediate formation of pyrans valence-isomeric with unsaturated carbonyl compounds in the pyridine syntheses based on reactions of ammonia with aldehydes or ketones, advocated by various authors (cf. Section II,B,2,f), is still rather speculative. (See also Section II,B,2,e for the valence isomerism of 5-chloro-2,4-dienones with pyrylium chlorides.)... 

Depending upon the substitution pattern, a thermal valence isomerization of 1,4-dioxocins 4 to the tricyclic jyn-benzene dioxides (xyn-3,8-dioxatricyclo[5.1,0.02-4]oct-5-enes) 3 can be detected. On the other hand, the valence isomerization of sin-benzene dioxides (anti-benzene dioxides do not undergo such rearrangements) provides a general approach to 1,4-dioxocins 4. 

Obviously, the advantages of this new synthetic method are i) valence isomerization can be performed under very mild conditions and ii) due to the fairly strong acidity of the bridgehead hydrogens of a bicyclobutane moiety, some substituents can be introduced regioselectively. Nevertheless, one serious disadvantage of this synthesis is the limitation of its applicability Thus, the formation of the precursor bicyclobutane derivatives is limited only to 46 and 56, and many attempts to prepare bicyclobutane precursors from other thiapyran derivatives have been unsuccessful so far. 

George and co-workershave investigated the reaction of cyclohexyl isocyanide (185) with DMAD and have shown that a major constituent of the product mixture is the 2 3 adduct (186) formed through a [6 -i- 4] addition of the initially formed intermediate (182) with the dipolar species (181, R = cyclohexyl). Thermal isomerization of 186 in refluxing xylene results in an isomeric spiro compound (187), whereas at higher temperatures, other valence isomers of 186 are formed (Scheme 29). - Winterfeldt had earlier isolated a 1 2 adduct (188) from the reaction of cyclohexyl isocyanide with DMAD. The reaction of some alkyl and aryl isocyanides with acetylenic esters in protic solvents, such as methanol, has been reported to give open-chain adducts with the incorporation of one or two solvent molecules. 

Valency isomerism is possible, depending on whether the bridging nitrogen of an amino-group is linked to one cobalt atom through a principal valency bond, and to the other cobalt atom by a subsidiary valence, as in the formula... 

Disubstitution (A and B H), on the other hand, destabilized intermediate conformations 460 and 462, causing then to be nonplanar and so facilitating their closure to a heterocycle (459). 165.167.174.2°8.395. .402 Cyclic forms 459 seem to exist only in cases where their valence isomeric cis dienones possess no stable planar conformation type of 460 or 462,164 but... 

Although valence isomerization reactions of aromatic compounds have found little by the way of practical application, they are a fascinating area for mechanistic and theoretical study. The details are not completely dear, but it seems that, for benzene itself, benzvalene arises from the lowest excited singlet state, perhaps by way of a biradical intermediate (3.32) that could also be a precursor to fulvene bicyclohexadiene is probably produced from the second excited singlet state. For some other aromatic compounds the electronic nature of 5, and S2 may be reversed, or at least the states are much closer in energy, so that the preference for benzvalene or bicyclohexadiene formation under conditions of long-wavelength irradiation can be rationalized. 

For other examples of valence isomerizations of small and large rings see Section 2.5.5.1. 

One final interesting preparation of this ring system involves the reaction of the tetrazolo[l,5-6]pyridazine (174) with polyphosphoric acid to give 7-azido-4H-pyrimido[l,2-6]pyridazin-4-one (175). This reaction involves azido-tetrazolo valence isomerization, which has been studied in other heterocyclic systems (71JHC1055). 

In this context it is useful to remember that the concept of the possible recombination of triplet radical ion pairs is not an ad hoc assumption to rationalize certain Z - E isomerizations, although the CIDNP effects observed during an isomerization reaction played a key role in understanding this mechanism. Triplet recombination has been accepted in several donor-acceptor systems as the mechanism for the generation of fast (optically detected) triplets [169-171], and invoked for several other reaction types [172]. The CIDNP technique is a sensitive tool for the identification of this mechanism, for example, in the geometric isomerization of Z- and E-1,2-diphenylcyclopropane and in the valence isomerization of norbornadiene (vide infra). Most of these systems have in common that the triplet state can decay to more than one minimum on the potential surface of the parent molecule. 

Anhydro-5-hydroxyoxazolium hydroxides lacking substituents at C(4) dimerize spontaneously by a process in which one molecule acts as an electrophile and the other as a nucleophile (Scheme 21). This accounts for the fact that dimeric products of this type are obtained by the action of dicyclohexylcarbodiimide on acylamino acids of the general formula R1C0NR2CH2C02H. Substituents at position 4 stabilize the mesoionic system the first compounds to be prepared were the acetyl derivatives (220) (B-49MI41800) and (221) (58Cl(L)46l) and much of the more recent work has been carried out with the relatively stable methyldiphenyl compound (222). This miinchnone decomposes above 115 °C to yield the allene (225) with loss of carbon dioxide. The mechanism proposed for this remarkable reaction (Scheme 22) involves valence isomerization to the ketene (223), which undergoes a 1,3-dipolar cycloaddition with the miinchnone. The product loses carbon dioxide to form a new betaine (224), which collapses to the allene as shown. 

We can next consider, as an annulene other than benzene, the very unstable enigmatic cyclobutadiene, see Refs. 94 and 95, which was first prepared some 40 years ago. The 13C NMR spectra from doubly labelled 13C molecules 97 suggest that the two valence isomeric forms square and rectangular, interconvert rapidly even at ca. 25 K. The energies and some NMR parameters of the two forms have been estimated recently from theory,98 as have those of benzene in its ground state—for comparison. More work is needed to establish exactly where the electron singlet and triplet states occur. 

Not only tautomeric equilibria are subject to considerable solvent effects. Other equilibria, such as rotational and conformational equilibria [81-83], cisitrans (or E/Z) isomerization, valence isomerization [84], ionization, dissociation, and association [85] (some of which are considered in Section 2.6), complex equilibria [86, 163, 262, 263], acid/base equilibria [264, 265] etc., are also strongly affected by the medium. Only a small number of representative examples will be considered in this Section in order to give an idea of how solvents can affect these different kinds of equilibria. 

Both the bidentate and monodentate processes probably play a role in the catalysis of symmetry-restricted reactions. For some reactions, hke the cyclobutanation of olefin Hgands suggested in olefin metathesis bidentate coordination is essential to the critical transformation. In other processes, such as the valence isomerization of quadricyclene 9) to norbornadiene (10) 10), both paths are available. 

The stepwise, oxidative cycloaddition mechanism [particularly with d metal systems 1 )] could intervene in the valence isomerizations of strained, cyclobutane ring systems where energy factors and difficulties in attaining bidentate coordination work in its favor. For the other processes, however, where bidentate coordination is either very favorable or guaranteed, its contribution to catalytic chemistry would seem to be significantly less. 

Another striking example of metal-assisted S5mimetry-forbidden valence isomerizations involves the silver ion catalyzed rearrangement of homo-cubyl systems 2) i.e., 58 - 5P). A similar rearrangement was reported for cubane itself using silver salts Interestingly, other metals [e.g., rho-... 

The silver ion, then, does not exhibit the same degree of back-bonding that the more familiar transition elements do. Since back-bonding is an essential factor in the forbidden-to-aUowed process and, in particidar, in direct oxidative addition, silver s function in this chemistry could differ. It may be that the silver ion (and other similar metallic species) stands apart from the other transition elements (W, Mo, Cr, Fe, Co, Ni, Rh, etc.) in its mode of catalysis. In the valence isomerization of quadricyclene, some oxidation occurs as evidenced by the deposition of metallic silver 45). Certainly, irreversible redox cannot be a feature of the actual catalytic path, since silver s role is definitely catalytic and the isomerization itself precludes it i.e., the oxidation state of the system remains fixed). Some electron transfer, however, clearly proceeds and may be a critical feature of the catalysis. One could speculate on the possibility of intermediate ion radicals generated through electron transfer from a reactant to Ag(I) followed by electron recapture by the rearranged species in the catal5dic system. 

Corresponding correlation diagrams for the benzene-prismane interconversion in Figure 7.45 explain why the direct irradiation of benzene does not produce prismane, while the reverse reaction is quite efficient. Correlation diagrams for the other benzene valence isomerization reactions can be derived in a similar way. (See also Halevi, 1977.)... 

Other examples of highly selective transition metal-catalyzed valence isomerization are reported by the Shell, Amsterdam workers (24). Ea o-tricyclooctene XV undergoes quantitative conversion to the tetra-cyclooctane XVI in the presence of Rh2(CO)4Cl2 at room temperature. 

Qualitative photochemical studies concentrate on diene complexes because of their greater thermal inertness and ease of characterization. Formation of olefin complexes is induced in situ either by photochemical or thermal means and their presence determined by spectroscopy. The photocatalyzed hydrogenation and hydrosilation of 1,3-di-enes the photocatalyzed valence isomerization of norbomadiene to quadricyc-lane and the cis trans photoisomerization of coordinated olefins are potentially usefulHowever, these transformations are not photosubstitution reactions and are not discussed here the reader should consult ref. 1 and references cited therein. Photolysis of olefin complexes leads to olefin loss with high quantum efficiency unless the olefin is a chelating di- or polyene where, as with most chelating ligands, other reactions occur. 

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